IPboard (invision) removed this feature. 3.0.5 was an old release, and the feture was embedded because most web browsers did not include a spell checker.

Now days, most all web browsers offer a spell checker which highlights misspellings as you type. If you are an internet explorer user, and you wish to have spellcheck, I believe that's the exception. To get spell check on I.E. you must download the "google toolbar" from google.

Again, the logic Invision was operating on was that a spell checker in IPboard, was no longer needed, due spellcheckers being a common feature on all modern browsers.